The typical tractor includes an endless drive belt rotatably mounted on the tractor body and driven by a drive sprocket; it may also rotate the belt about a spaced idler sprocket. The drive sprocket is driven by a drive shaft, and rotation causes the drive belt to push or pull the web. There is a rectilinear path for the belt which is tangential to both sprockets and extends therebetween. Most usually, the tractor belt has drive pins on its outer surface which engage in perforations along the sides of the web as they move upwardly about the entrance sprocket, and exit the perforations as the web leaves the tractor as it goes beyond the exit sprocket.
Sheet-feed tractors are typically used in pairs, one for each edge of the web. A support shaft spaced from the drive shaft extends through a spaced opening in the tractor body, and the pair of tractors are clamped to the support shaft at the spacing necessary to allow the pins on the belts to register simultaneously with the perforations in the sheet material.
It is important that the pins cleanly leave the pins of the belt at the exit sprocket to avoid wear on the pins through excessive frictional engagement with the web as the web slides off the pins and to avoid distortion of the perforations in the web.
Particularly in tractors which rapidly and intermittently move the web, or which rapidly move it backwards as well as forwards, both common in printers, it is desirable to provide some slack in the belt between its sprocket supports to absorb some of the forces resulting from the rapid reversal or termination of drive motion. However, it is also desirable to avoid excessive sagging of the belt between the guide surfaces to ensure that the belt holds the web tightly against the cover and firmly seated on the drive pins. Moreover, it is desirable to avoid having the lower surface of the web bear upon the upper surface of the tractor chassis between the sprockets.
It is an object of the present invention to provide an improved tractor with a novel mechanism for adjusting the tension on the drive belt.
It is also an object to provide such a tractor in which the web is moved off the pins on the drive belt in a manner to reduce frictional wear on the pins and distortion of the perforations in the web.
Another object is to provide such a tractor in which the components may be fabricated readily and relatively economically and assembled to provide a long lived tractor.